In manufacturing tubing such as seamless steel tubes, it is necessary to accurately measure and gauge the wall thicknesses at several points on the periphery of a tubular object, in a non-contacting manner. One such apparatus and method is disclosed in U.S. patent application Ser. No. 190,800, filed Sept. 25, 1980, now U.S. Pat. No. 4,393,305, entitled "A Method and Apparatus for Measuring Tube Wall Thickness", said application being assigned to the assignee of the present application. The method and apparatus disclosed in the latter referenced patent application, while novel, still have disadvantages, wherein improvements are possible.
Firstly, the prior method and apparatus is not as compact in arrangement of the measuring equipment as may be preferable in some cases. The prior apparatus includes plural sets of the measuring instruments with each set comprising a radiation source and a detector, and specifically the number of instrument sets required is at least equal to the number of the measuring points (c.f. the description in the former application), since the same number of radiation beams must be produced to pass through those measuring points.
In general, instruments using radioactive rays should be provided with a shield of a relatively large thickness of about 5 or 10 cm. When using X-rays, the required radiation source is usually at least as massive as a source for radioactie rays. In the former apparatus and method, plural sets of such instruments including the radiation sources with containers (or the shields) are required to be arranged within a rather narrow circular zone around the tube to be examined. Consequently, when many measuring points are used, the equipment of the former invention may be quite complex in arrangement of the instruments, with the result that difficulty may be experienced in assembly or maintenance, particularly when the equipment is used in on-line applications.
Secondly, the former invention is not well suited to applications where one desires to frequently change the diameter of the tubes to be measured. In those cases, the plural measuring instruments which are stationarily mounted around a tube have to be removed and repositioned when the diameter of one or more tubes to be measured is changed.
For example, in FIG. 1 there is illustrated in solid lines a measuring equipment arrangement having seven radiation beams (i.e. seven measuring points) around a tube 20S which has a relatively small diameter. Each measuring equipment set comprises a radiation source container 2 and radiation detector 4. When the tube to be measured is changed to another tube 20L which has a relatively larger diameter as shown by broken lines, the seven measuring points (i.e. points of intersection of the lines) move outwards. Therefore the seven sets of measuring instruments have to change their positions in a manner which is typically shown by broken lines for two of the seven. Changing the positions of those instruments is not particularly easy. Further, it may eventually cause the instruments to butt against each other in part, as shown by reference character Z in FIG. 1. In order to avoid having to reposition these instruments, it is necessary to have a sufficient distance between the radiation source 2 and detector 4 of each set of the measuring instrument. This results in a large space being required for equipment, with a corresponding increase in capacity for the radiation source, and consequently a larger shield for the source.
Thirdly, the former invention is directed to a case where a value k, equal to an actual transit path length S of radiation beam across a tube wall through a measuring point, divided by the wall thickness x at that point, is not adequately approximated, not easily determined nor given beforehand. If the expected range of variation, or the unevenness of outer and inner surfaces of a measured tube are small, then no particular problem arises at obtaining an accurate k value. However, there may be other cases where the value k=S/x is either not given beforehand, nor easily determined, nor obtainable by any adequate approximation. Accordingly, a solution to the problem of obtaining an accurate k value would be desirable.
Referring to FIG. 2, a value of k, or the relation between a radiation beam transit path length and tube wall thickness, could be obtained by using the following equations: ##EQU1## wherein x is the radial thickness in a direction of a radial line OA passing through the center of the tube section and a measuring point B, l is a line representing a radiation beam, .xi. is an oblique thickness of the tube wall along the line l (i.e. the length of the segment of the line 1, whose endpoints are defined by the outer and inner surfaces of the tube), R.sub.1 and R.sub.2 are outer and inner radii respectively of the tube, R.sub.0 is a radius of a circle passing through the measuring point B and with its center at the center 0 of the tube section, and h is a height of the perpendicular from the center 0 to the line 1. The values of O and R.sub.0 are known. A value of .xi. can be determined, provided that both R.sub.1 and R.sub.2 are known, in Eq. (2), or provided that both R.sub.1 and x are known, in Eq. (3). In the case where the unevennesses of both the outer and inner surfaces of a tube are neglible, the value of k, i.e. the ratio of .xi. to x can be obtained. But otherwise, it generally cannot be obtained.
Fourth, preventing the radial deflection of a tube can further improve the accuracy of measurement in the prior art apparatus and method. If a tube is motionless when it is being measured, no significant problem may result. However, if a tube is running at a high speed, for example, during an actual process of manufacturing seamless steel pipes, and particularly where an on-line measurement is required, any radial deflection or vibration may cause deflections of relative positions of radiation beams and measuring points to the tube, resulting in errors in measurement which are no longer negligible.
Radial deflection of a tube may be caused, for example, by a kind of random shifting of feed line of the tube, by a bend of the tube, or by distortion of the tube from a genuine circular shape.
It may be possible to detect horizontal and vertical deflections of the axis of a running tube by photoelectrical or flying image sensor means, for example, and to dynamically shift the position of the measuring equipment so as to follow the deflecting tube position. However, this requires a costly device. Also, it may be possible to make corrections of measured results according to the detected tube axis deflection. However, this still requires a costly device or a complicated program of computer. FIG. 3 shows an example of the relation between the one-directional tube axis deflection .delta. (abscissa, in mm) and the corresponding correction necessary in the measured output. It is .epsilon. obtained by computer simulation, assuming that the tube outer diameter is 300 mm, its wall thickness 9.93 mm, and the radiation beam thickness 10 mm, and their arrangement and the tube deflection direction are as shown in FIG. 4, where MP1 through MP3 denote measuring points, and .delta. denotes the deflection. In FIG. 3, lines 1, 2 and 3 indicate corrections required at the points MP1, MP2 and MP3, respectively. The required accuracy for measuring wall thickness of a tube is normally about 0.1 mm. However, considering that deflection of the tube axis can occur further in modes other than that shown in FIGS. 3 and 4, adequate correction for deflection is impractical. Therefore, some other means of improvement are desirable.